builtin-rerere: more carefully find conflict markers

When a conflicting file contains a line that begin with "=======", rerere
failed to parse conflict markers. This result to a wrong preimage file and
an unexpected error for the user.  The boundary between ours and theirs
not just begin with 7 equals, but is followed by either a SP or a LF.

This patch enforces parsing rules so that markers match in the right order,
and when ambiguous, the command does not autoresolve the conflicted file.

Especially because we are introducing rerere.autoupdate configuration
(which is off by default for safety) that automatically stages the
resolution made by rerere, it is necessary to make sure that we do not
autoresolve when there is any ambiguity.

Signed-off-by: Olivier Marin <dkr@freesurf.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-rerere.c

@@ -112,11 +112,17 @@ static int handle_file(const char *path,
 	strbuf_init(&one, 0);
 	strbuf_init(&two,  0);
 	while (fgets(buf, sizeof(buf), f)) {
-		if (!prefixcmp(buf, "<<<<<<< "))
+		if (!prefixcmp(buf, "<<<<<<< ")) {
+			if (hunk)
+				goto bad;
 			hunk = 1;
-		else if (!prefixcmp(buf, "======="))
+		} else if (!prefixcmp(buf, "=======") && isspace(buf[7])) {
+			if (hunk != 1)
+				goto bad;
 			hunk = 2;
-		else if (!prefixcmp(buf, ">>>>>>> ")) {
+		} else if (!prefixcmp(buf, ">>>>>>> ")) {
+			if (hunk != 2)
+				goto bad;
 			if (strbuf_cmp(&one, &two) > 0)
 				strbuf_swap(&one, &two);
 			hunk_no++;
@@ -142,6 +148,10 @@ static int handle_file(const char *path,
 			strbuf_addstr(&two, buf);
 		else if (out)
 			fputs(buf, out);
+		continue;
+	bad:
+		hunk = 99; /* force error exit */
+		break;
 	}
 	strbuf_release(&one);
 	strbuf_release(&two);

t/t4200-rerere.sh

@@ -9,6 +9,8 @@ test_description='git rerere
 . ./test-lib.sh
 
 cat > a1 << EOF
+Some title
+==========
 Whether 'tis nobler in the mind to suffer
 The slings and arrows of outrageous fortune,
 Or to take arms against a sea of troubles,
@@ -24,6 +26,8 @@ git commit -q -a -m initial
 
 git checkout -b first
 cat >> a1 << EOF
+Some title
+==========
 To die, to sleep;
 To sleep: perchance to dream: ay, there's the rub;
 For in that sleep of death what dreams may come
@@ -35,7 +39,7 @@ git commit -q -a -m first
 
 git checkout -b second master
 git show first:a1 |
-sed -e 's/To die, t/To die! T/' > a1
+sed -e 's/To die, t/To die! T/' -e 's/Some title/Some Title/' > a1
 echo "* END *" >>a1
 git commit -q -a -m second
 
@@ -55,14 +59,14 @@ test_expect_success 'conflicting merge' '
 
 sha1=$(sed -e 's/	.*//' .git/rr-cache/MERGE_RR)
 rr=.git/rr-cache/$sha1
-test_expect_success 'recorded preimage' "grep ======= $rr/preimage"
+test_expect_success 'recorded preimage' "grep ^=======$ $rr/preimage"
 
 test_expect_success 'rerere.enabled works, too' '
 	rm -rf .git/rr-cache &&
 	git config rerere.enabled true &&
 	git reset --hard &&
 	! git merge first &&
-	grep ======= $rr/preimage
+	grep ^=======$ $rr/preimage
 '
 
 test_expect_success 'no postimage or thisimage yet' \
@@ -71,7 +75,7 @@ test_expect_success 'no postimage or thisimage yet' \
 test_expect_success 'preimage has right number of lines' '
 
 	cnt=$(sed -ne "/^<<<<<<</,/^>>>>>>>/p" $rr/preimage | wc -l) &&
-	test $cnt = 9
+	test $cnt = 13
 
 '
 
@@ -80,13 +84,23 @@ git show first:a1 > a1
 cat > expect << EOF
 --- a/a1
 +++ b/a1
-@@ -6,17 +6,9 @@
+@@ -1,4 +1,4 @@
+-Some Title
++Some title
+ ==========
+ Whether 'tis nobler in the mind to suffer
+ The slings and arrows of outrageous fortune,
+@@ -8,21 +8,11 @@
  The heart-ache and the thousand natural shocks
  That flesh is heir to, 'tis a consummation
  Devoutly to be wish'd.
 -<<<<<<<
+-Some Title
+-==========
 -To die! To sleep;
 -=======
+ Some title
+ ==========
  To die, to sleep;
 ->>>>>>>
  To sleep: perchance to dream: ay, there's the rub;
@@ -124,7 +138,7 @@ test_expect_success 'another conflicting merge' '
 '
 
 git show first:a1 | sed 's/To die: t/To die! T/' > expect
-test_expect_success 'rerere kicked in' "! grep ======= a1"
+test_expect_success 'rerere kicked in' "! grep ^=======$ a1"
 
 test_expect_success 'rerere prefers first change' 'test_cmp a1 expect'